1. Field of the Invention
The present invention concerns wavelength-division multiplex fiber optic transmission systems and more particularly the regeneration of signals therein.
2. Description of the Prior Art
It is well known that to transmit optical signals over very long distances, as in the case of transoceanic transmission by submarine cable, it is necessary to amplify the optical signals periodically to compensate the attenuation of the signal and to reshape the signals to compensate distortion induced by the transmission medium or by interaction between the various signals of the multiplex. The distortion is compensated by “3R” regenerators (“3R” signifying “reshaping”, “retiming” and “reamplifying”) whose functions include reshaping the pulses, retiming the pulses and compensating pulse intensity losses in the regenerator.
Wavelength-division multiplexed (WDM) transmission systems are tending to include more and more channels (or wavelengths) on each optical fiber. The bit rate per channel is also increasing. Thus the most recent transoceanic transmission systems have a capacity of 32 channels each of 10 Gbit/s.
There are various ways to regenerate the signals of each channel in such systems. One solution is to demultiplex the signals and to regenerate each signal individually. However, this solution has the disadvantage of requiring as many regenerators as there are multiplexed signals which, given the trend for the number of channels to increase, leads to a complex and physically large regenerator, also requiring a powerful electrical power supply, which represent heavy penalties in the case of transmission via submarine cable.
Another solution which can be used in the case of soliton WDM signals (or RZ signals converted into solitons at the regenerator input) consists in using a regenerator including means for compensating chromatic dispersion in order to resynchronize the various channels followed by a synchronous modulator. This solution has the disadvantage of requiring very accurate control of chromatic dispersion so that the signals are perfectly synchronized at the input of the synchronous modulator. Controlling the chromatic dispersion is all the more difficult in that it must be achieved for a large number of channels.
It has also been proposed, in the case of WDM soliton signals, to dispose the regenerators along the optical line at locations where a certain number of channels are naturally synchronous, and to regenerate only these channels at each of these locations (see WO-A-98 35459). However, this technique introduces a constraint on the position of the regenerators which can sometimes be problematical, depending on the number of channels and their wavelength spacing.
The invention proposes a simple solution to the problem of regenerating channels in a WDM transmission system, and one which remains simple even if the number of channels is high.